1. Technical Field
The present invention relates to a surface acoustic wave (SAW) resonator and particularly to a SAW resonator suitable for apparatuses that perform communications by a frequency shift keying (FSK) system.
2. Related Art
Conventionally, as a digital signal communication system, an FSK communication system is known. The FSK system is, for example, a modulation system that discretely changes an instantaneous frequency of a carrier wave in response to a digital signal of 0 and 1 and is a communication system that is capable of modulating while keeping an amplitude change constant and is not much affected by the level fluctuation or noise. Further, the circuitry structure of the FSK system is simple, and its bandwidth during communication is wider than that in an amplitude shift keying (ASK) system or in a phase shift keying (PSK) system.
Therefore, the FSK communication system is highly usable, and various suggestions have been made on oscillators using this type of communication system. For example, oscillators proposed in Japanese Patent No. 2925158 and JP-A-2004-40421 also use the FSK communication system.
The oscillator disclosed in Japanese Patent No. 2925158 includes a plurality of SAW resonators having different resonant frequencies on a single piezoelectric substrate and oscillation circuits whose number corresponds to the number of the SAW resonators on a single semiconductor integrated circuit substrate, while the oscillation circuits are connected to a switching circuit.
The oscillator having such a structure operates the switching circuit in accordance with the frequency band to be used and selects an oscillation circuit coupled to the SLAW resonator in accordance with a, desired frequency band for oscillation.
Further, an oscillation circuit depicted in JP-A-2004-40421 is equipped with, in the circuit, two types of SAW filters having different frequency bands to be passed and a switching circuit on the output side of the SAW filters, and is composed of an amplifier and a feedback circuit having a fixed phase shifter.
With such an oscillator, only a required frequency of the frequency bands can be selected as the switching circuit switches the connected SAW filters.
The oscillator and oscillation circuit depicted in Japanese Patent No. 2925158 and JP-A-2004-40421 can provide the oscillator and oscillation circuit in accordance with a desired frequency band.
However, the oscillator and oscillation circuit depicted in Japanese Patent No. 2925158 and JP-A-2004-40421 are both equipped with a plurality of SAW resonators or SAW filters having different frequency bands and respond to the required frequency band by switching the SAW resonators or the SAW filters according to the frequency band to be used. Thus, when switching the SAW resonators or the SAW filters, errors such as instantaneous interruption of the resonant frequency and discontinuity of an output phase may occur due to such factors as electrical connection and phase difference between the SAW resonators or the SAW filters. Problems such as these are particularly influential in high-speed communications. Further, because the oscillation circuit depicted in Japanese Patent No. 2925158 includes the plurality of SAW resonators of different uses in a single circuit, it is not possible to obtain frequency accuracy and frequency deviation precision in every SAW resonator.